This invention relates to a process of forming a transistor with three vertical gate electrodes and the resulting transistor. By forming such a transistor it is possible to maintain an acceptable aspect ratio as MOSFET structures are scaled down to sub-micron sizes. The transistor gate electrodes can be formed of different materials so that the workfunctions of the three electrodes can be tailored. The three electrodes are positioned over a single channel and operate as a single gate having outer and inner gate regions.

Technique to mitigate short channel effects with vertical gate transistor with different gate materials

Thickness measurement of SiO2 films thinner than 1 nm by X-ray photoelectron spectroscopy

Very exciting and promising results from recent developments in group-IV alloy heterostructures (viz., SiGe, SiGeC, SiC, GeC and strained-Si) have led to the belief that SiGe-based devices will open up an entirely new dimension to the future of VLSI/ULSI technology. The growth of ultrathin dielectric films on a strained group-IV alloy layer is a challenging task. As metal-oxide-semiconductor devices are being aggressively scaled down, high permittivity dielectrics are being widely investigated as alternative gate insulating layers in advanced MOS devices. The present paper reviews the recent results of different gate and high-k dielectrics on group-IV alloy layers for scaled CMOS devices, high-mobility pure-Ge channel devices and nanocrystal floating gate memories.

High-k gate oxide for silicon heterostructure MOSFET devices

Polycrystalline silicon germanium (Si1−xGex) films deposited by the rf sputtering technique were characterized using a combination of transmission electron microscopy (TEM), atomic force microscopy (AFM), x-ray diffraction (XRD), and Raman spectroscopy techniques. The TEM results showed small grains (10–20 nm) with microtwins, and AFM showed islands of 100–200 nm in the films. The XRD results show that our films consist of Si1−xGex alloy with no cluster of Ge or a Ge-rich material embedded in a Si matrix. The smaller grains in our films could be a result of an abundance of nucleating sites or impurities in the films. Raman spectroscopy results indicate that our films were strain free.

Microstructural characterization of rf sputtered polycrystalline silicon germanium films

In the study of ultra-thin films (<10-nm thick), there is a range of methods that can provide accurate measurements of differences in thickness. However, in a pilot study under the auspices of the Consultative Committee for Amount of Substance (CCQM), results for the archetypal system of SiO2 on Si show that the methods have different offsets such that, at all thicknesses, positive or negative amounts in the range up to 1 nm may be observed between methods. All the methods studied give thicknesses that are greater than those measured by X-ray photoelectron spectroscopy (XPS) by amounts between 0.2 nm and approximately 1 nm. Significant parts of these offsets, of other methods with respect to XPS, may be attributed to contaminations which increase the apparent thickness but that do not affect XPS. However, not all of these offsets can yet be explained at the 0.2 nm level. The remaining part of the offsets could be thought to have arisen either from the XPS or from the other methods. In this study, by measuring SiO2 deposited in situ on amorphous Si by XPS it is shown that the XPS linearity is consistent with the previous estimate of ± 0.025 nm, down to fractions of a monolayer, with no significant offset and that, therefore, it is the offsets seen using other methods that need further study. Recent calculations of the film thickness dependence of the attenuation lengths (ALs) for this system, using NIST SESSA software, are not consistent with these data although earlier calculations are. This work shows that XPS, with the AL calibrated by one or more other methods that are valid for differences in thickness, can provide a traceable measurement of thickness in all laboratories. © Crown Copyright 2007. Reproduced with the permission of Her Majesty's Stationery Office. Published by John Wiley & Sons, Ltd.

Ultra-thin SiO2 on Si VIII. Accuracy of method, linearity and attenuation lengths for XPS

Polysilicon films have been deposited at a low temperature by single and dual ion beam sputtering The structural and electrical properties of as-deposited and annealed films have been characterized by x-ray diffraction, secondary ion mass spectroscopy, atomic force microscopy, and Hall mobility measurements The films are microcrystalline with average grain size ranging from 200 to 400 A The films exposed to a low-energy secondary ion beam during sputtering from a silicon target have exhibited smoother surface topography and different electrical behavior than the films deposited without any secondary ion bombardment Some preliminary studies on ion beam sputtered SiGe films using a compound target are also presented

Ion assisted growth and characterization of polycrystalline silicon and silicon-germanium films

Ion-assisted oxidation has been carried out at a substrate temperature of 150 °C for fabrication of metal-oxide-semiconductor capacitors using fully strained Si1-xGex (x = 0.2 and 0.26) and partially strained Si1-x-yGexCy (Ge:C = 20:1 and 40:1) films. The oxide thickness is designed to consume a part of the Si-cap layer, leading to the formation of a valence band quantum well for confining the carriers away from the Si-SiO2 interface. The fixed oxide charge density and mid-gap interface trap density are found to be higher for the sample containing higher Ge and C concentration. The presence of hole-trapping centres is observed for all the samples. However, a decrease in the hole trap density is observed either with the reduction of Ge content or with the increase of carbon fraction in the film.

Ion-assisted low-temperature oxidation for fabrication of strained Si1-xGex and Si1-x-yGexCy MOS capacitors

Polysilicon films have been deposited at a low temperature by single and dual ion beam sputtering. The structural and electrical properties of as-deposited and annealed films have been characterized by X-ray diffraction, secondary ion mass spectroscopy, atomic force microscopy and Hall mobility measurements at room temperature as well as low temperature region (290°K-70°K). The study revealed that the films are microcrystalline with average grain size ranging from 200 A to 400 A. The films exposed to low energy secondary ion beam during deposition has exhibited smoother surface topography and better electrical properties than that of films deposited without any secondary ion bombardment. Some preliminary results on ion beam sputtered SiGe films using a composite target (Si, Ge) have also been presented.

Ion Assisted Growth and Characterization of Polycrystalline Silicon and Silicon-Germanium Films

Microwave plasma oxidation of polycrystalline silicon has been carried out to grow thin polyoxide films at a low temperature in ambient. The electrical properties of grown oxides have been studied using a metal-oxide-semiconductor structure. The current-voltage characteristics of polyoxides have been studied for different poly doping and post-oxidation annealing conditions. It is found that the polyoxide on undoped poly-Si has lower leakage current and higher breakdown voltage, whereas the oxide on doped poly-Si has higher tunnelling efficiency at lower field. The oxide on boron doped poly-Si exhibits electron trapping behaviour.

Chumki Saha

Papers

Ion assisted growth and characterization of polycrystalline silicon and silicon-germanium films

Ion-assisted low-temperature oxidation for fabrication of strained Si1-xGex and Si1-x-yGexCy MOS capacitors

Ion Assisted Growth and Characterization of Polycrystalline Silicon and Silicon-Germanium Films

Electrical properties of thin polyoxides grown at a low temperature using microwave oxygen plasma